Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric (boost pressures). A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to the intake manifold of the engine, thereby increasing engine power.
The turbocharger shaft is conventionally supported by journal and thrust bearings, including appropriate lubricating systems, located within a central bearing housing connected between the turbine and compressor wheel housing. It is well known that providing an effective sealing system to prevent oil leakage from the central bearing housing into the compressor or turbine housing is problematic. Oil leakage is regarded as a particular problem at the compressor end of the turbocharger since at low boost pressures (e.g. when the engine is idling) there can be a significant drop in pressure from the bearing housing to the compressor housing which encourages oil leakage into the compressor housing. It is for instance conventional to include an oil slinger in the compressor end seal assembly. An oil slinger is an annular component which rotates with the turbocharger shaft and has surfaces or passages arranged for propelling oil away from the shaft as it rotates, and in particular from the passage through the bearing housing into the compressor housing.
It is moreover important for a sealing arrangement to be able to withstand the increasingly high boost pressures that are delivered by modern turbochargers. The pressure of the bearing housing is effectively at the same pressure as the engine oil sump (typically around up to 100 millibar) and there is thus a pressure gradient between the bearing housing and the compressor and turbine housings. A significant pressure differential can also be set up between the bearing and turbine housing when an exhaust gas engine braking arrangement is employed at the turbine outlet as this creates back pressure. The sealing arrangement must thus be able to restrict “blow-by” gas flow from the relatively high pressure regions of the turbine and compressor housings into the relatively low pressure region of the bearing housing, without impairing its ability to restrict oil leakage.
Although oil leakage at the turbine end of the turbocharger is regarded as less of a problem, it is nevertheless important to prevent oil leaking into the turbine housing where it will mix with the exhaust gas and increase exhaust emissions. The turbocharger turbine wheel is conventionally welded to a seal boss at the end of the turbocharger shaft. In existing turbocharger bearing housings sealing is typically provided by one or more sealing rings (a piston-type rings) seated in annular grooves defined in an outer surface of the shaft or in the housing and which serve to restrict gas flow between the housing and the shaft in a direction in or out of the bearing housing. A rotating surface perpendicular to the shaft is provided to sling oil and so control oil leakage.
In addition to the blow-by gases referred to above, as the sealing efficiency of the engine piston rings decreases additional blow-by exhaust gases enter the crankcase and increase its internal pressure. Such an increase in crankcase pressure is transmitted to the bearing housing of the turbocharger as the two are connected by virtue of the bearing oil from the turbocharger draining back to the crankcase. Exhaust gases can thus enter the bearing housing, become entrained with the oil and “blow-by” the turbine end seal. With increasingly stringent exhaust emission regulations there are restrictions on venting such gases to atmosphere and there is a continuing need to improve the efficiency of the turbine end seal arrangement.
An example of a known oil seal arrangement is shown in our European patent application published under no. EP 1387061.
Rotating hydraulic seals for sealing rotating shafts by making use of the centrifugal force acting on the hydraulic fluid have been known for some time. An example is disclosed in U.S. Pat. No. 1,014,850. In short, an annular channel defined by a fixed structure is penetrated by a thin disc or plate defined on the rotating shaft (or vice versa). Oil is maintained in the channel by centrifugal forces generated by the rotating disc or channel and provides a seal between the two. However, such seals have not been thought suitable for use in the hot environment of a turbocharger. The very high rotation speeds of a turbocharger (up to 150,000 r.p.m.) would cause extremely high shear rates in the sealing oil and thereby raise its temperature to a level where there is a risk of breakdown by coking.